Investment gains/losses - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule Of Gain Loss On Investments Including Marketable Securities And Investments Held At Cost Income Statement Reported Amounts Summary [Line Items]
Sales and redemptions of equity securities			$ 15,700	$ 4,500
Losses on securities sold	$ (87)	$ 5	(11,233)	194
Taxable gains (loss) on sales of equity securities	$ (4,500)	$ 832	(3,300)	$ 1,350
Market Value Changes in First Quarter [Member]
Schedule Of Gain Loss On Investments Including Marketable Securities And Investments Held At Cost Income Statement Reported Amounts Summary [Line Items]
Losses on securities sold			$ (10,700)